An exhaust gas containing an organic nitrogen compound or ammonia (hereinafter referred to as a nitrogen-based exhaust gas) frequently has a foul odor, and is often harmful to the human body. Thus, its treatment is demanded.
Precious metal catalysts such as platinum or palladium, have hitherto been used for the treatment of the nitrogen-based exhaust gas. These catalysts have high decomposition activity for organic nitrogen compounds and ammonia, but have high selectivity for NOx, such as NO and NO2. Thus, the resulting NOx needs to be further reduced into N2 with the use of denitrification catalysts.
The following examples have been reported as catalysts with high N, selectivity. Firstly, there is a report of a catalyst in which a metal such as Cu has been sulfated and supported on a silica carrier (see patent document 1). With this catalyst, however, the metal has to be kept sulfated during the reaction, so that a volatile sulfur compound needs to be added to the exhaust gas.
Next, there is a report of a catalyst having a vanadium oxide, a tungsten oxide, and palladium carried on titania and/or titania silica, and it is also described that a copper compound can be used Instead of the vanadium oxide and the tungsten oxide (patent document 2). However, the use of the copper compound is considered to result in poor activity, and a further improvement is demanded.
Moreover, a catalyst having a Cu compound supported on carriers such as Al2O3 and zeolite it also described (see patent document 3). This catalyst is deemed to convert acrylonitrile to N2 at a high selection rate, but there is a demand for a catalyst which has a higher N2 selection rate and which can be prepared by a simple method.
Furthermore, a catalyst for purification of NOx, which contains copper oxide, ZSM5 and/or zeolite β, and an oxide of at least one element among magnesium, calcium and silver, is reported for treatment of NOx in an exhaust gas of a diesel engine (see patent document 4). When this catalyst is applied to the decomposition of an organic nitrogen compound, the problem arises that the production of CO and NH3, which are by-products, increases and the rate of conversion Into N2 decreases.
Patent document 1: Japanese Unexamined Patent Publication No. 1996-173766
Patent document 2: Japanese Unexamined Patent Publication No. 2001-293480
Patent document 3: Japanese Unexamined Patent Publication No. 2004-58019
Patent document 4: Japanese Unexamined Patent Publication No. 2004-167306